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Bash 中的方括號和雙括號有什麼不同？

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Bash 中的方括號和雙括號有什麼不同？

我注意到在這個問題一位回答者使用雙括號，而另一位回答者則使用方括號：

if (( $(fileSize FILE1.txt) != $(fileSize FILE2.txt) )); then

…

if [ $(fileSize FILE1.txt) != $(fileSize FILE2.txt) ]; then

我以前沒見過雙括號 - 谷歌搜尋也沒有幫助。它們的意義完全相同嗎？便攜性有什麼差別嗎？優先選擇其中一個的理由是什麼？

答案1

從bash線上說明頁：

   ((expression))
          The  expression is evaluated according to the rules described below under ARITHMETIC EVALUATION.  If the value of the expression is
          non-zero, the return status is 0; otherwise the return status is 1.  This is exactly equivalent to let "expression".

   [[ expression ]]
          Return a status of 0 or 1 depending on the evaluation of the conditional expression expression.  Expressions are  composed  of  the
          primaries  described  below  under  CONDITIONAL  EXPRESSIONS.  Word splitting and pathname expansion are not performed on the words
          between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process  sub‐
          stitution, and quote removal are performed.  Conditional operators such as -f must be unquoted to be recognized as primaries.

例如，您可以用來(( ))執行數學比較和位元比較，以及[[ ]]執行更抽象的比較（與）test file attributes and perform string and arithmetic comparisons

touch test;
if [ -e test ]; then
    echo test exists
else
    echo test does not exist
fi

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